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    <title>Document</title>
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    <SCript>
        /*
        以下这种异步回调函数嵌套异步回调函数的写法被称作为回调地狱
        回调地狱耦合度过高，不方便维护和阅读
        */
        console.log('请求A数据...');
        setTimeout(() => {
           console.log("A数据请求成功");
           console.log("请求B数据...");
           setTimeout(() => {
              console.log("B数据请求成功");
              console.log("请求C数据...");
              setTimeout(() => {
                  console.log("C数据请求成功");
              }, 1000); 
           }, 2000); 
        }, 3000);
    </SCript>
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